Packet loss on peer associations? - NTP

This is a discussion on Packet loss on peer associations? - NTP ; All,
I have recently started peering two of my NTP servers. One is a primary
server, the other only has Internet-based sources of synchronisation.
They are attached to the same LAN, which is lightly loaded, as are the
servers themselves ...

Packet loss on peer associations?

All,

I have recently started peering two of my NTP servers. One is a primary
server, the other only has Internet-based sources of synchronisation.
They are attached to the same LAN, which is lightly loaded, as are the
servers themselves (CPU usage rarely exceeds 1% and there is little disk
activity).

Quite often though, the association misses a beat (e.g. the reach number
reported by ntpq -p is 376 or another combination of mostly 1s and few
0s). I would expect to see this in the presence of packet loss, but I
really don't think that that is the case here.

Note that I have clamped both maxpoll and minpoll to 6 on both sides
(because otherwise the problem was even worse, whenever the two peers
did not agree on the poll interval).

Is this a problem?

(One server is running ntpq 4.2.3p41@1.1401-o under Linux 2.4.20 on a
P4, the other ntpq 4.2.2p4@1.1585-o under Linux 2.4.31 on a soekris Geode).

Thanks, Jan

Re: Packet loss on peer associations?

> All,
>
> I have recently started peering two of my NTP servers. One is a primary
> server, the other only has Internet-based sources of synchronisation. They
> are attached to the same LAN, which is lightly loaded, as are the servers
> themselves (CPU usage rarely exceeds 1% and there is little disk
> activity).
>
> Quite often though, the association misses a beat (e.g. the reach number
> reported by ntpq -p is 376 or another combination of mostly 1s and few
> 0s). I would expect to see this in the presence of packet loss, but I
> really don't think that that is the case here.
>
> Note that I have clamped both maxpoll and minpoll to 6 on both sides
> (because otherwise the problem was even worse, whenever the two peers did
> not agree on the poll interval).
>
> Is this a problem?
>
> (One server is running ntpq 4.2.3p41@1.1401-o under Linux 2.4.20 on a P4,
> the other ntpq 4.2.2p4@1.1585-o under Linux 2.4.31 on a soekris Geode).
>
> Thanks, Jan

I have the same question with a similar setup. I have two Stratum 2s peered
with each other. Both are Linux machines running on the same LAN with
little traffic on the LAN itself, both have light system loads, both are
running ntp 4.2.4, and each have their own sychronization sources. One
server is a pool server and the other server is not visible to the internet.
Running 'ntpq -p' on each server routinely shows its peer with a reach of
376, while the rest of the synchronization sources show 377. I also don't
believe packet loss is an issue here. Is this typical behavior with peered
servers?

Thanks,

Dennis

Re: Packet loss on peer associations?

Jan,

It is normal that one or the other peers in symmetric modes appear to
lose a packet. Not so; it might happen that the poll intervals don't
completely match and one peer occasiionally transmits two packets while
the other one transmits one. The protocol exchange is specially designed
to detect what appear to be duplicate, missing or out of order packets.

If the poll intervals happen to be different, the protocol selects the
minimum value of the two. If you leave things alone and the intervals
finally climb to 1024 s while mismatching along the way, no harm is done.

Dave

Jan Ceuleers wrote:
> All,
>
> I have recently started peering two of my NTP servers. One is a primary
> server, the other only has Internet-based sources of synchronisation.
> They are attached to the same LAN, which is lightly loaded, as are the
> servers themselves (CPU usage rarely exceeds 1% and there is little disk
> activity).
>
> Quite often though, the association misses a beat (e.g. the reach number
> reported by ntpq -p is 376 or another combination of mostly 1s and few
> 0s). I would expect to see this in the presence of packet loss, but I
> really don't think that that is the case here.
>
> Note that I have clamped both maxpoll and minpoll to 6 on both sides
> (because otherwise the problem was even worse, whenever the two peers
> did not agree on the poll interval).
>
> Is this a problem?
>
> (One server is running ntpq 4.2.3p41@1.1401-o under Linux 2.4.20 on a
> P4, the other ntpq 4.2.2p4@1.1585-o under Linux 2.4.31 on a soekris Geode).
>
> Thanks, Jan